The present invention relates to submersible motor pump assemblies, namely, to assemblies wherein the pump casing and the motor housing are flooded with liquid which is conveyed by the pump. More particularly, the invention relates to improvements in apparatus for preventing penetration of solid impurities into the housing of the submersible motor (also called underwater motor or U-motor if the conveyed liquid is water), especially during starting and acceleration of the motor to normal operating speed.
It is already known to construct submersible motor pump assemblies (hereinafter called assemblies for short) in such a way that the pump casing and/or the motor housing provides a path for the flow of liquid from the casing into the housing. The path normally includes an annular clearance between a tubular member, which connects the casing with the housing, and the peripheral surface of the pump shaft which latter is driven by the motor and transmits torque to the impeller or impellers of the pump. In other words, the just described conventional assemblies do not employ a stuffing box around the pump shaft in the region between the impeller or impellers of the pump and the stator of the motor. Such assemblies can be used for circulation of liquid in a boiler. It is also known to provide a conventional assembly with means for directing a stream of flushing liquid from the interior of the motor housing toward the interior of the pump casing so that the stream prevents penetration of solid impurities into the motor. As a rule, the flushing liquid is pure cool water so that such liquid can serve the additional purpose of preventing overheating of the motor. In many instances, the stream of flushing liquid is caused to flow from the motor housing toward and into the casing of the pump only during starting of an assembly wherein the pressure of pumped liquid during starting is relatively low. However, it is also known to operate with a stream of flushing liquid in assemblies wherein the pressure of pumped liquid is high or relatively high in the course of the starting operation. One of the presently accepted classifications of pumps according to pressure is that between high-pressure pumps with a nominal total head between 200 and 1200 m, low pressure pumps with a nominal total head not exceeding 80 mm, medium-pressure pumps with a nominal total head ranging between 80 and 200 m, and very-high-pressure pumps with a nominal total head in excess of 1200 m. When the total head is relatively low, the flushing liquid can be furnished by the condensate conveying system. When the total head is higher and the flushing is to take place at elevated pressures, flushing liquid is supplied by the boiler feed pump. Since the permissible operating temperature of the motor is limited, flushing liquid which is supplied by a boiler feed pump must be cooled prior to admission into the motor housing; furthermore, such liquid must be cleaned in order to ensure that it does not entrain solid or other impurities into the interior of the motor.
A drawback of the above-described and other apparatus for preventing penetration of solid impurities into the motor housing of an assembly is that they are complex and expensive. Thus, such apparatus necessitate the utilization of pipelines, valves, cooling systems and filters. Furthermore, conventional apparatus for preventing penetration of solid impurities into the motor housing are far from being foolproof, i.e., they are highly likely to permit contamination of the motor housing in the event of a malfunction of the assembly and/or when the assembly is operated by an unskilled, semiskilled or careless attendant. Penetration of solid impurities into the motor housing is highly likely to entail rapid destruction of or, at the very least, extensive damage to component parts (especially bearings) of the motor.
The likelihood of damage to component parts of the motor in a submersible motor pump assembly is especially pronounced when the pump shaft is vertical and the motor is installed at a level below a centrifugal pump. Such assemblies are often utilized in boiler plants. Solid impurities, especially products of corrosion consisting of or containing a high percentage of magnetite, are highly likely to be circulated by the pump during starting as well as subsequent to testing of the boiler plant. The increased percentage of magnetite and/or other impurities in the pump circuit during the just mentioned stages of operation of the assembly is highly likely to entail contamination of the motor, especially since the assembly is normally located at the lowermost point of the closed system for circulation of the fluid to be pumped. Thus, when the motor is turned off, corrosion products tend to migrate toward and to accumulate, in large quantities, in the casing of the centrifugal pump forming part of the motor pump assembly. When the motor is started again, a substantial percentage of solid impurities which have accumulated in the pump casing is likely to penetrate into the motor housing barring effective measures for prevention of contamination of the motor. In the absence of such measures, the motor housing is contaminated in the following way:
The pump establishes or develops a pressure differential between the interior of the pump casing and the interior of the motor housing in response to starting of the motor. The pressure in the interior of the pump casing rises and propagates into the clearance between the interior of the pump casing and the interior of the motor housing. As explained above, the just mentioned clearance is formed between the internal surface of a sleeve-like or tubular portion of the pump casing or motor housing (i.e., a portion of the housing or enclosure of the assembly) and the peripheral surface of the pump shaft which extends from the housing of the motor and upwardly into the interior of the pump casing to drive the impeller means of the pump. Even though the motor is normally provided with automatic air evacuating means or is designed with a view to exhibit a self-venting feature, at least some air is highly likely to remain entrapped between the motor winding and the package of stator laminations. Such residual air is compressed in response to rising pressure in the pump casing and the propagation of pressure into the motor housing by way of the clearance around the pump shaft. This enables a certain quantity of liquid (namely a quantity filling a volume corresponding to that by which the volume of entrapped or residual air is compressed in the interior of the motor housing) to penetrate into the motor housing. The liquid which flows into the motor housing is laden with solid impurities, i.e., such impurities penetrate into the interior of the motor housing and can lead to serious damage to or total destruction of bearings and/or other component parts of the motor. The pressure differential between the interior of the pump casing and the interior of the motor housing is reduced to zero only after the impeller means of the pump rotates at the normal or full speed, i.e., when the air compressing step is terminated. In other words, liquid ceases to flow from the pump casing into the motor housing only with a certain delay after starting, i.e., when the RPM of the pump shaft has risen to the maximum value.